1. Brief Description of the Invention
The invention relates to an improved process for preparing acetals, especially water- and alcohol-free acetals. Acetals are solvents and intermediates in the chemical industry.
2. Related Art
Acetals can be prepared by acid-catalyzed reaction of alcohols with aldehydes. One industrially important acetal is methylal (formaldehyde dimethyl acetal). It is prepared industrially by acid-catalyzed reaction of aqueous formaldehyde with methanol. It is possible to remove methylal from the reaction mixture by distillation, but it is accompanied by water and methanol because the two components each form a binary azeotrope with methylal. For the solution of this separation problem, numerous processes have been proposed; most of them utilize extraction or extractive rectification in order to overcome the azeotropes.
DE 1 002 305 discloses a process for extractive distillation of water- and methanol-containing methylal with water in order to obtain methanol-free methylal.
CH 688 041 describes a process for preparing methylal from formaldehyde and methanol by reactive distillation. The reaction takes place in a distillation column with catalysis by an acidic ionic exchange resin which has been introduced into one or more column sections in suitable form. At the top of the column, the azeotrope of methylal and methanol is obtained and is worked up in the further three separating steps                extraction with aqueous alkali,        rectification to give the azeotrope with 0.9-1% water,        final drying to remove water with an inorganic desiccant such as calcium chloride, zeolites or silica gel,to obtain approx. 99.99% pure methylal. The process has the disadvantage that, for the alkaline extraction and for the final drying, assistants have to be introduced and have to be disposed of or worked up. The reactive distillation which is advantageous in itself is followed by three further separation steps before the pure product.        
U.S. Pat. No. 6,379,507 discloses a process for preparing methylal from formaldehyde and methanol, according to which the particular mixture of water, formaldehyde, methanol and methylal is withdrawn at at least four positions in a distillation column and the mixtures are each fed in forced flow to the corresponding at least four fixed bed reactors. The fixed bed reactors charged with an acidic ion exchange resin are dedicated vessels separate from the column. The apparatus demands for a plurality of separate reactors with dedicated pump and control system are considerable. The process attains a concentration of 0.3-0.4% methanol and 1.2% water at the top of the column, but only with use of a defoamer which is fed to the top of the column.
U.S. Pat. No. 6,015,875 describes a process for preparing acetals from formaldehyde and methanol by reactive distillation at elevated temperature and pressure approx. 7 bar, the acidic catalysts used being zeolites. However, the process achieves only 73-77% methylal at the top of the reactive column.